<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .btn {
            background-color: brown;
        }
    </style>
</head>

<body>
    <button class="btn">按钮1</button>
    <button>按钮2</button>
    <button>按钮3</button>
    <button>按钮4</button>
    <script>
        let btns = document.querySelectorAll('button')
        for (let i = 0; i <= btns.length; i++) {
            btns[i].addEventListener('click', function () {
                document.querySelector('.btn').className = ''
                this.className = 'btn'
            })
        }
    </script>
</body>

</html>